The growth hormones from man and from the common domestic animals are proteins of approximately 191 amino acids, synthesized and secreted from the anterior lope of the pituitary gland. Human growth hormone consists of 191 amino acids.
Growth hormone is a key hormone involved in the regulation of not only somatic growth, but also in the regulation of metabolism of proteins, carbohydrates and lipids. The major effect of growth hormone is to promote growth.
The organ systems affected by growth hormone include the skeleton, connective tissue, muscles, and viscera such as liver, intestine, and kidneys.
Until the development of the recombinant technology and the cloning of the growth hormone gene now giving rise to production of e.g. human growth hormone (hGH) and Met-hGH in industrial scale, human growth hormone could only be obtained by extraction from the pituitary glands of human cadavers. The very limited supplies of growth hormone restricted the use thereof to longitudinal growth promotion in childhood and puberty for treatment of dwarfism, even though it has been proposed for inter alia treatment of short stature (due to growth hormone deficiency, normal short stature and Turner syndrome), growth hormone deficiency in adults, infertility, treatment of burns, wound healing, dystrophy, bone knitting, osteoporosis, diffuse gastric bleeding, and pseudoarthrosis.
Furthermore, growth hormone has been proposed for increasing the rate of growth of domestic animals or for decreasing the proportion of fat.
Pharmaceutical preparations of growth hormone tend to be unstable. Degradation products such as deamidated or sulfoxydated products and dimer or polymer forms are generated—especially in solutions of growth hormone.
The predominant chemical degradation reactions of hGH are 1) deamidation by direct hydrolysis or via a cyclic succinimide intermediate to form various amounts of L-asp-hGH, L-iso-asp-hGH, D-asp-hGH, and D-iso-asp-hGH (ref 1-3), 2) oxidation of the methionine residues in positions 14 and 125 (ref 4-9), and 3) cleavage of peptide bonds.
Deamidation especially takes place at the Asn in position 149.
hGH is rather easily oxidized in positions 14 and 125, especially in solution (4-8), since the oxidation of hGH in solution forming sulfoxides is normally due to the oxygen dissolved in the preparation.
At present, it is not believed that these degradation products should have toxic or altered biological activity or receptor binding properties, but there is indication to the effect that the conformation stability of the sulfoxides is reduced as compared to native hGH.
Other “degradation products” of growth hormone are aggregation products such as dimers and polymers. Studies have been performed to clarify the role of these forms in inducing an immune response with measurable amounts of antibodies to native hGH. These studies have indicated that aggregates of hGH are the primary cause of immunogenicity in patients.
Thus, it is desirable in a pharmaceutical formulation to avoid the formation of aggregates of growth hormone since such aggregates are capable of causing undesirable immunogenicity or altered half-life.
The kinetics of degradation depend on temperature, pH and various additives or adjuvants in the hGH formulation.
Due to the instability, a growth hormone formulation is at present normally lyophilized and stored in the lyophilized form at 2-8° C. until it is reconstituted for use in order to minimize the degradation. The lyophilized pharmaceutical preparations comprising hGH are reconstituted by the patient and then stored as a solution during the use for a period of up to 30 days, during which some degradation will take place.
It is at present preferred to reconstitute the growth hormone as late as possible before use and to store and ship the preparation in a lyophilized state. The chain from the manufacturer to the pharmacy is apt for handling the preparations at a controlled low temperature of e.g. 2-8° C. which allows for a shelf life of up to three years.
Preferably, a lyophilized and then reconstituted preparation should be stable with the end user in a lyophilized state for about one month and additionally for one month in a reconstituted state in a pen device for the intended period of use of a cartridge.
As an alternative to lyophilized and reconstituted preparations, growth hormone may be formulated as a liquid formulation suitable for use in vials or in a pen system for self-medication. The extended use of pen systems for self-medication and the expanded field of use calls for a preparation which is stable for a sufficient long time with the end user. Such stabilization is of very great importance when moving the administration of the growth hormone from clinics to the homes of the individuals to be treated where optimal storage conditions may not be available. A “ready to use” formulation furthermore diminishes any handling problems in connection with reconstitution and thus represents a convenience for the patient.
A stable dissolved preparation comprising growth hormone may be produced ready to use in the form of vials used by the patient in combination with conventional syringes or as cartridges fitting into the pen device used by the patient. In both cases, the patient may then avoid the reconstitution of the preparation and, hence, will not have to be in the possession of a lyophilized preparation, a suitable vehicle for reconstitution as well as the necessary skill and sterile equipment for sterile reconstitution of the preparation. Safety reasons also make it desirable to avoid the reconstitution of a lyophilized preparation just before the use of the preparation.
From the manufacturer's point of view, it may be an advantage to avoid the lyophilization step in the production of growth hormone preparations. Lyophilization is a time consuming and costly process and is also often a “bottleneck” in the production due to the limited capacity of the freeze drier.
Thus, there is a need for more stable dissolved preparations of growth hormone in order to facilitate the handling to be performed by the patient.
Thus, there is also a need to reduce the rate of the degradation processes in order to allow for dissolved hGH preparations being stable during shelf life and during the period of use of up to about one month.